Risk-based insulation coordination studies for protection of medium-voltage overhead lines against lightning-induced overvoltages

This paper presents the performance evaluation of various lightning protection schemes of medium-voltage overhead lines using risk-based insulation coordination method. The flashover characteristics of the line insulation have been modelled by normal probability distribution. The peak lightning-induced overvoltages have been evaluated using Agrawal's coupling model. The probability distribution of peak lightning-induced overvoltages is determined by applying Monte Carlo simulation method considering the probability distributions of the peak stroke current, return stroke velocity and the distance between overhead line and striking point. Based on the computational results, it has been demonstrated that the lightning performance of overhead lines can be improved by reducing the spacing between two consecutive surge arresters to less than 200 m. On the other hand, the application of shield wire in distribution networks is quite limited. However, the combination of shield wire and surge arresters is used in some regions with high lightning density. Thus, the possibility of adopting the surge arresters in combination with shield wire to mitigate induced overvoltages flashover is also evaluated in this work. It has been demonstrated that larger spacing between surge arresters may be allowed if they are employed in combination with shield wire. Accordingly, the alternate grounding of shield wire in conjunction with surge arresters is proposed in this paper. In this way, the line designer can minimize the risk of insulation flashover below the tolerable limit and protect the line with lower number of surge arresters.